Which of the following routes is associated with the fastest onset of action?

Inhalation is associated with the fastest onset of action among the given routes of administration due to the direct delivery of the drug to the lungs, where it can enter the bloodstream swiftly. When a drug is inhaled, it bypasses the gastrointestinal tract and avoids first-pass metabolism in the liver, which can delay the effects of a medication. The alveoli in the lungs provide a vast surface area for gas exchange, allowing for rapid absorption of drugs into the circulatory system. This characteristic makes inhalation particularly effective for medications that require quick relief, such as those used for asthma or respiratory distress.

The other routes listed involve more complex absorption processes. For example, oral administration requires the drug to be dissolved in the gastrointestinal fluids, absorbed through the intestinal wall, and then metabolized, which can take considerable time. Rectal administration, while offering a quicker option than oral in certain cases, still may not achieve the rapid systemic effect seen with inhalation. Transdermal delivery involves absorption through the skin and is designed for slow and sustained release rather than immediate effect, making it unsuitable for conditions requiring rapid intervention.